Partial oxidations of organic substrates have always been commercially very important and for this purpose no other reagent could be as popular, as useful and as successful as the chromium (VI) reagents have been. Though there are some limitations of chromium reagents in terms of environmental antagonism, the popularity of such reagents does not seem to diminish because their performance under mild conditions with high efficiency and cost-effectiveness weigh far over their limitations. Thus, chromium (VI) based oxidizing agents have been extensively worked on leading to the development of a good number of reagents, many of which have become quite popular and performing well as oxidizing agents. Some of the important entries in the list of chromium (VI) reagents are the Collin's reagent, chromium-trioxide-3,5-dimethyl-pyrazole complex, pyridinium chlorochromate (PCC), pyridinium fluorochromate (PFC), pyridinium dichromate (PDC), quinolinium fluorochromate (QFC) and 2,2′-bipyridinium chlorochromate (BiPCC). In addition, there are a few other systems which work as reagents in combination with chromium (VI), e.g., chromium (VI)-t-butylhydroperoxide, chromium (VI)-peroxocarbonate and chromium (VI)-peroxoborate.
Pyridinium fluorochromate (PFC), in addition to being a vital reagent for several biochemically important syntheses, is a highly efficient oxidant for a wide variety of oxidative organic transformations. As oxidant, it is widely used in the oxidation of alcohols, hydroxy acids, allylic, benzylic, substituted toluenes, fused ring hydrocarbons, organic sulfides, benzyl ethers, phosphorus compounds, thioacids, substituted mandelic acids, oxidative deprotection of oximes and desilylative oxidation of alkyl trimethylsilyl ethers. PFC also acts as a very powerful oxo transfer agent such as conversion of PPh3 to OPPh3.
PFC successfully exercises its role as an imperative reagent for the synthesis of hydroxy ethylene, ketomethylene dipeptide isosteres, S-(+)-4-Formyl-4-Butanolide, (R)-1-benzoyloxy-3-buten-2-ol, derivatives of dimethyl penam and dimethyl penam-S, S-dioxide, sex pheromones of pink bollworm. Yet another manifestation of its versatility stems from its use for the investigations of a wide variety of reaction dynamics. Thus for instance PFC has been widely used in the kinetics and mechanistic studies of the reactions discussed above.
Pyridinium fluorochromate, often referred to as PFC is well known in the art as witnessed for example by its well deserved incorporation in the Encyclopedia of Reagents for Organic Synthesis, L. A. Paquette (Ed. in chief), 1995, Vol. 6, p. 4369, John Wiley and Sons, Inc., New York and Reagents for Organic Synthesis, M. Fieser, 1984, Vol. 11, p. 453, John Wiley and Sons, Inc., New York. Reference is made to Synthesis, 1982, 588; Bull. Chem. Soc. Jpn., 1984, 57, 2019 and Inorg. Synth. 1990, 27, 310 wherein chromium trioxide is dissolved in water and aqueous hydrofluoric acid and to the solution pyridine is added. The solution is then heated to yield the crystals of PFC. The disadvantages of the procedure are the use of an excess of hydrofluoric acid and necessity of heating. Reference is made to J. Fluorine Chem., 1997, 81, 211; wherein ammonium bifluoride (NH4HF2) is dissolved in water followed by the addition of CrO3 and to the clear orange solution pyridine is added. The clear orange solution is heated to yield the crystals of PFC. The disadvantage of the procedure is that the product is formed in very poor yield. Extra preparation of NH4HF2 is required adding to a relatively high cost of production.